1. Technical Field
The present invention relates to a voltage converter controller and a voltage converter circuit and, more particular to a voltage converter controller and a voltage converter circuit with voltage-drop compensation.
2. Description of Related Art
Please refer to U.S. Pat. No. 8,143,845 (hereinafter referred to as patent '845) for the prior arts of the skill. Patent '845 discloses a voltage converter circuit of a topology of flyback switching voltage converter with compensating function on a voltage drop caused by a parasitic resistance of a conducting line in a charging circuit. Thus, when charging a battery as a load of the voltage converter circuit, a charging voltage received by the battery can still be maintained within a range defined by system specification with the varied charging current. The voltage converter circuit of patent '845 takes advantage of the characteristics that an output voltage of an error amplifier thereof is positive-correlated to the charging current. By adjusting the reference voltage according to the output voltage of the error amplifier, correspondingly an output voltage of the voltage converter circuit is changed for partly compensating the voltage drop of the conducting line. Furthermore, the error amplifier disclosed in patent '845 was designed to be with finite voltage gain resulting in a limited loop DC gain, which causes a steady-state error on the output voltage of the voltage converter circuit to further compensate the voltage drop of the conducting line. However, such design will result in poor output resistance performance of the voltage converter circuit. Additionally, the variation of the performance of the compensation will be somewhat larger when observed in mass production case.
FIG. 1 is a circuit block diagram of a voltage converter circuit 100 with voltage-drop compensation of another prior art. The voltage converter circuit 100 is a flyback switching voltage converter which converts an input voltage on the converting input terminal 101 to an output voltage on a converting output terminal 103 through a transformer 102 which includes primary windings 1021 and secondary windings 1022. The converting output terminal 103 provides a load current 111 to a load 120 by electric connection through a conducting line 110. The conducting line 110 can be a charging cable and the load 120 can be a rechargeable battery; that is, in this case the voltage converter controller 110 provides a power and charges the load 120 through the conducting wire 110. However, there is a parasitic resistance on the conducting line 110 which causes a voltage drop when the load current 111 flows through the conducting line 110, resulting in difference between the output voltage on the converting output terminal 103 and the charging voltage on the load 120. Because of hardware limitation in most of charging applications, the terminal where the load 120 receives the charging voltage cannot be taken as a feedback point to perform more accurate regulation on the charging voltage. Instead, the converting output terminal 103 is often taken as the feedback point in most applications, as shown in FIG. 1. Therefore, although the converting output terminal 103 taken as the feedback point can be relatively accurate thereon, the charging voltage received by the load 120 will be largely varying accompanying variations of the load current 111 and the parasitic resistance determined by the length and material choices of the conducting line 110. In worst cases, the charging current may not comply with system specifications such as within plus/minus 5-percent range of the rated voltage value.
However in the related art, it is well known that a voltage on the output terminal of an error amplifier is positive-correlated to the load current in a switching voltage converter of current-mode control. For example, the voltage converter circuit 100, as a switching voltage converter of current-mode control adopts sensing resistor 104 to sense current on a conducting channel of a power switch 105 and to convert it to a voltage signal being fed-back to a comparator 131 to be compared with an output voltage of an error amplifier 132, resulting in a controlling mechanism for cutting off the channel of the power switch 105. From a first-ordered analysis, it is found that in the voltage converter circuit 100, the output voltage of the error amplifier 132 is proportional to a square root of the load current 111. Hence, by detecting the output voltage of the error amplifier 132 to deduce the load current 111, the voltage on the converting output terminal 103 can be manipulated to maintain the charging voltage received by the load 120 to comply with system specifications.
For example, in the voltage converter circuit 100, a transconductance stage 133 is adopted with an input terminal and an output terminal thereof coupled to the output terminal of the error amplifier 132 and a feedback terminal 106 respectively. A voltage on the feedback terminal 106 is generated by dividing the voltage on the converting output terminal 103 by a voltage divider composed of a first feedback resistor 107 and a second feedback resistor 108. The transconductance stage 133 multiplies a voltage on the input terminal thereof by a transconductance and generates correspondingly a compensating sink current coupling to the feedback terminal 106. Therefore, when the load current 111 is larger, the voltage on the output terminal of the error amplifier 132 is correspondingly higher and the transconductance stage 133 generates a larger compensating sink current to the feedback terminal 106 resulting in a higher steady-state voltage on the converting output terminal 103 to compensate for the larger voltage drop on the conducting line 110 and maintain the voltage received by the load 120 within the specified range. On the contrary, when the load current 111 is smaller, the voltage on the output terminal of the error amplifier 132 is correspondingly lower and the transconductance stage 133 generates a smaller compensating sink current to the feedback terminal 106, resulting in a lower steady-state voltage on the converting output terminal 103 to compensate for the smaller voltage drop on the conducting line 110 and still maintain the voltage received by the load 120 within the specified range.
According to first-ordered analysis, it is established that when the transconductance Gm1 of the transconductance stage 133 follows equation (1), the optimized voltage-drop compensating effect on the converting output terminal 103 can be derived to maintain the variation, due to varying load current, of the voltage received by the load 120 in a smallest range:
                                          G                          m              ⁢                                                          ⁢              1                                =                                                                                          L                    P                                    ⁣                                      ·                                          I                      o                                                                                        2                  ⁢                                                                          ⁢                                      Vo                    ·                                          T                      S                                                                                            ·                                                            V                  ref                                ·                                  R                  cab                                                            Vo                ·                                  R                  S                                                      ·                          (                                                1                                      R                    a                                                  +                                  1                                      R                    b                                                              )                                      ,                            (        1        )            where Lp is an effective inductance of the primary windings 1021, Io is the load current 111, Vref is a reference voltage 134, which determines the steady-state voltage of the feedback terminal 106, Rcab is an effective resistance of the conducting line 110, Vo is the output voltage when the load current 111 is zero, Ts is a modulating period of the pulse-width modulation performed in the voltage converter circuit 100, Rs is a resistance of the sensing resistor 104, Ra is a resistance of the first feedback resistor 107, and Rb is a resistance of the second feedback resistor 108.
From equation (1) it can be observed that the optimized value of Gm1 is correlated to the load current 111. Thus, if a constant transconductance irrelevant to the load current 111 is designed for the transconductance stage 133, the conditions of over-compensating and/or under-compensating will happen under different load current 111 and result in a degraded effect on the voltage-drop compensation. Even the voltage received by the load 120 still cannot meet system specification by the compensation in this way.